Electric vehicles (EVs) have been actively studied because they are the most promising solution to pollution and energy problems.
Electric vehicles (EVs) are mainly powered by an AC or DC motor using battery power. Electric vehicles are broadly classified into battery powered electric vehicles and hybrid electric vehicles. In the battery powered electric vehicles, a motor is driven using power of a battery, and the battery is recharged after stored power is consumed. In hybrid electric vehicles, a battery is charged with electricity generated via engine driving, and an electric motor is driven using the electricity to realize vehicle movement.
The hybrid electric vehicles may further be classified into serial and parallel types. In the case of serial hybrid electric vehicles, mechanical energy output from an engine is changed into electric energy via a generator, and the electric energy is fed to a battery or motor. Thus, the serial hybrid electric vehicles are always driven by a motor similar to conventional electric vehicles, but an engine and generator are added for the purpose of increasing range. Parallel hybrid electric vehicles may be driven using two power sources, i.e. a battery and an engine (gasoline or diesel). Also, the parallel hybrid electric vehicles may be driven using both the engine and the motor according to traveling conditions.
With recent gradual development of motor/control technologies, small high-output and high-efficiency systems have been developed. Owing to replacing a DC motor by an AC motor, electric vehicles have accomplished considerably enhanced output and power performance (acceleration performance and maximum speed) comparable to those of gasoline vehicles. As a result of promoting a higher output and higher revolutions per minute, a motor has achieved reduction in weight and size, and consequently reduction in the weight and size of a vehicle provided with the motor.
A typical vehicle is configured to operate internal loads using a small-sized battery, or starts operation using energy of a small-sized battery. Although the electric vehicle includes a high-voltage battery for operating the vehicle, a small-sized auxiliary battery is discharged upon completion of starting of the electric vehicle. If a battery voltage reaches a discharge stop voltage, the battery is no longer used and it is impossible to resume the electric vehicle system.